ASP.NET MVC 2 -- ValidateAntiForgeryTokenAttribute for JSON requests

ValidateJsonAntiForgeryTokenAttribute.cs

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public class ValidateJsonAntiForgeryTokenAttribute : FilterAttribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationContext filterContext)
    {
        if (filterContext == null)
        {
            throw new ArgumentNullException("filterContext");
        }

        var httpContext = new JsonAntiForgeryHttpContextWrapper(HttpContext.Current);
        filterContext.HttpContext = httpContext;

        new ValidateAntiForgeryTokenAttribute().OnAuthorization(filterContext);
    }

    public string Salt
    {
        get;
        set;
    }

    private class JsonAntiForgeryHttpContextWrapper : HttpContextWrapper
    {
        private readonly HttpRequestBase _request;

        public JsonAntiForgeryHttpContextWrapper(HttpContext httpContext)
            : base(httpContext)
        {
            _request = new JsonAntiForgeryHttpRequestWrapper(httpContext.Request);
        }

        public override HttpRequestBase Request
        {
            get
            {
                return _request;
            }
        }
    }

    private class JsonAntiForgeryHttpRequestWrapper : HttpRequestWrapper
    {
        private readonly NameValueCollection _form;

        public JsonAntiForgeryHttpRequestWrapper(HttpRequest request)
            : base(request)
        {
            _form = new NameValueCollection(request.Form);

            if (request.Headers["__RequestVerificationToken"] != null)
            {
                _form["__RequestVerificationToken"] = request.Headers["__RequestVerificationToken"];
            }
        }

        public override NameValueCollection Form
        {
            get
            {
                return _form;
            }
        }
    }
}